Higher operating temperatures for gas turbine engines are continuously being sought in order to improve their efficiency. However, as operating temperatures increase, the high temperature durability of the components of the engine must correspondingly increase. Significant advances in high temperature capabilities have been achieved through the formulation of iron, nickel, and cobalt-based superalloys. While superalloys have found wide use for components used throughout gas turbine engines, and especially in the higher temperature sections, alternative lighter-weight component materials have been proposed.
Ceramic matrix composites (CMCs) are a class of materials that consist of a reinforcing material surrounded by a ceramic matrix phase. Such materials, along with certain monolithic ceramics (i.e. ceramic materials without a reinforcing material), are currently being used for higher temperature applications. These ceramic materials are lightweight compared to superalloys, yet can still provide strength and durability to the component made therefrom. Therefore, such materials are currently being considered for many gas turbine components used in higher temperature sections of gas turbine engines, such as airfoils (e.g. turbines, and vanes), combustors, shrouds and other like components, that would benefit from the lighter-weight and higher temperature capability these materials can offer.
CMC and monolithic ceramic components can be coated with environmental barrier coatings (EBCs) to protect them from the harsh environment of high temperature engine sections. EBCs can provide a dense, hermetic seal against the corrosive gases in the hot combustion environment, which can rapidly oxidize silicon-containing CMCs and monolithic ceramics. Additionally, silicon oxide is not stable in high temperature steam, but is converted to volatile (gaseous) silicon hydroxide species. Thus, EBCs can help prevent dimensional changes in the ceramic component due to such oxidation and volatilization processes. Unfortunately, there can be some undesirable issues associated with standard, industrial coating processes such as plasma spray and vapor deposition (i.e. chemical vapor deposition, CVD, and electron beam physical vapor deposition, EBPVD) currently used to apply EBCs.
A typical air plasma spray (APS) microstructure for a rare earth disilicate is porous in the deposited state and is not hermetic toward the gaseous species that cause volatilization of the ceramic matrix composite. Therefore, a glassy layer such as barium strontium alumino-silicate (BSAS) is sprayed to provide a hermetic layer toward these gaseous species. However, this glassy layer cannot contact a silicon source such as silicon or silica and thus an additional layer of rare earth silicate must be sprayed to separate the glassy layer from the silica source. FIG. 1 is an illustration of an example of a prior art EBC having non-hermetic rare earth silicate-based layers in combination with BSAS.
Accordingly, there remains a need for environmental barrier coatings to protect CMCs from the high temperature steam environments present in gas turbine engines.
The present system and techniques are directed to overcoming these and other deficiencies in the art.